Control of Speed and Torque
The speed of a DC motor is a direct result of the voltage applied. As indicated
earlier, the DC motor requires two separate circuits to generate
motor torque.
Control of Speed
The field receives voltage from a separate power supply, sometimes
referred to as a field exciter. This exciter provides power to the field, which
in turn generates current and magnetic flux. In a normal operating state,
the field is kept at maximum strength, thereby allowing the field winding
to develop maximum current and flux. This condition is known as opera-
tion in the armature range. (The only way to control the speed is through
change in armature voltage.)
The armature power supply applies voltage to the armature through the
brushes and the commutator. Basically, the greater the amount of voltage
applied, the faster the speed of the motor. We can see this relationship in
the formula below:
where:
| | S | = | speed in rpm |
| | Va | = | armature voltage |
| | Ia | = | armature current |
| | Ra | = | resistance of the armature |
| | K1 | = | motor design constant |
| | φ | = | strength of the field flux |
As seen in the formula, if the load on the motor remains constant, the
armature current will stay constant, as well as the resistance of the armature.
In addition, the motor design constant will remain the same, as well
as the strength of the field flux. When all of these components remain constant, the only determining factor in speed is the amount of armature
voltage applied.
The above formula will work in determining speed, when at or below the
base speed of the motor. The formula will also indicate speed, when operating
above base speed. It is possible to operate in an extended speed
range, as long as the motor manufacturer is consulted for the maximum
safe operating speed.
As shown in the formula, if armature voltage is at maximum and all the
other components remain constant, speed can possibly be increased by
reducing the field flux (φ). It is necessary to point out, however, that this
must be done with caution.
Reduced field flux is the result of reducing the voltage from the field
exciter. If voltage is reduced to near zero, the speed of the armature can
increase to the point of motor self-destruction. This operation above base
speed is known as the field weakening speed range, for apparent reasons. The
field exciter will have safeguards in place to avoid excessive speed. Most
DC drive systems will allow a field weakening range of no less than 1/3 of
the normal voltage. If the voltage drops to less than that amount, pre-programmed
safety circuits in the drive shut down the armature supply and
bring the motor to a safe stop.
Increased speed is made possible by a reduced amount of field flux, when
operating above base speed. In essence, less EMF is available to act as
holdback magnetic flux. Torque available from the motor is also a function
of speed.
Typical armature voltage ratings in the United States are 90, 180, 240, or
500 VDC. Typical U.S. field voltage ratings are 100, 200, 150, or 300 VDC.
As stated earlier, the amount of voltage applied to the armature would dictate
the output shaft speed. For example, if a 1750-rpm motor with a 240
VDC armature had 120 VDC applied (1/2 voltage), the shaft speed would
be approximately 875 rpm (1/2 speed).
Control of Torque
Under certain conditions, motor torque remains constant when operating
below base speed. However, when operating in the field weakening range,
torque drops off inversely as 1/speed2. The amount of motor torque can
also be determined by a formula. The following relationship exists in a DC
motor and serves to help determine the motor torque available:
where:
| | T | = | torque developed by the motor |
| | K1 | = | motor design constant |
| | φ | = | strength of the field flux |
| | Ia | = | armature current |
As seen in the formula, if the field flux is held constant, as well as the
design constant of the motor, then the torque is proportional to the armature
current. The more load the motor sees, the more current is consumed
by the armature.
A selling point of DC motors is their ability to provide full torque at zero
speed. This is accomplished by the two power supplies, energizing their
power structures to supply voltage to the armature and field. When additional
load is dropped across the armature, magnetic flux of the armature
cuts through the field flux. Once this occurs, more current is drawn
through the armature, and the drive's power structure conducts the
required amount of current to meet the demand. This phenomenon occurs
whether the motor is at any speed, including zero.
Control of Speed and Torque
The speed of a DC motor is a direct result of the voltage applied. As indicated
earlier, the DC motor requires two separate circuits to generate
motor torque.
Control of Speed
The field receives voltage from a separate power supply, sometimes
referred to as a field exciter. This exciter provides power to the field, which
in turn generates current and magnetic flux. In a normal operating state,
the field is kept at maximum strength, thereby allowing the field winding
to develop maximum current and flux. This condition is known as opera-
tion in the armature range. (The only way to control the speed is through
change in armature voltage.)
The armature power supply applies voltage to the armature through the
brushes and the commutator. Basically, the greater the amount of voltage
applied, the faster the speed of the motor. We can see this relationship in
the formula below:
where:
| | S | = | speed in rpm |
| | Va | = | armature voltage |
| | Ia | = | armature current |
| | Ra | = | resistance of the armature |
| | K1 | = | motor design constant |
| | φ | = | strength of the field flux |
As seen in the formula, if the...
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